Tabular silver halide grains are crystals possessing two major faces that are substantially parallel. The average diameter of said faces is at least three times the distance separating them (the thickness). This is generally described in the art as an aspect ratio of at least 3:1.
Silver halide photographic emulsions containing a high proportion of tabular grains have advantages of good developability, improved covering power and increased useful adsorption of sensitizing dye per weight of silver due to their high surface area-to-volume ratio. The use of such emulsions in photographic elements is disclosed in U.S. Pat. Nos. 4,425,425, 4,425,426, 4,433,048, 4,435,499, 4,439,520, and other related patents.
The use of automatic processors for the rapid processing (i.e., for a processing of from 45 to 90 sec) of light-sensitive silver halide elements including tabular silver halide grains, in particular light-sensitive silver halide elements for radiographic use, is known. Such elements generally include a support (usually provided with a very thin subbing layer) having coated on at least one side thereof a silver halide gelatin emulsion layer coated in turn with a gelatin protective layer. These elements are transported through the machine processing units (developing, fixing, washing and drying) by means of opposed or staggered rollers (as described, for example, in U.S. Pat. No. 3,025,779) which also have the function of squeezing liquid from the film prior to drying. In recent years the increased use of silver halide elements for radiography has led to a strong request for a reduction of processing times. If rapid processing of a film takes place, several problems can occur, such as an inadequate image density (i.e. insufficient sensitivity, contrast and maximum density), insufficient fixing, insufficient washing, and insufficient film drying. Insufficient fixing and washing of a film cause a progressive worsening of the image quality and modification of the silver tone. In order to reduce the time taken by the element to pass through the processing machine from 2 to 0.5 minutes, as particularly required in rapid processing of radiographic elements, the processing is performed at relatively higher temperatures, usually higher than 30.degree. C., preferably between 35-45.degree. C., such as 38.degree. C., and the gelatin content of the silver halide emulsions is considerably reduced as compared to that of emulsions for manual processing.
Under such conditions, even with the changes in the emulsions, the physical and photographic properties of the elements processed in an automatic processor tend to be worse. With high temperatures and in presence of such low gelatin content, for instance, the intrinsic sensitivity to pressure of the silver halide grains gets higher and the elements processed in the automatic processor show marks caused by the pressure of the transporting rollers.
In order to prevent pressure marking, various methods have been described in the art. To this purpose, U.S. Pat. No. 2,960,404 describes the use in the photographic elements of glycerine, ethylene glycol and the like, Japanese Pat. No. 5316/1972 describes the use of 1,4-cyclohexane dimethanol and the like, and Japanese Pat. No. 4939/1978 describes the use of trimethylol propane. Another possible method of preventing pressure marking is by increasing the degree of hardening of the gelatin layers, in particular of the external protective layers. As another method, photographic elements are known wherein an intermediate gelatin layer is interposed between the support and the emulsion layer.
For example, U.S. Pat. No. 3,637,389 describes a rapid processing photographic element wherein gradation, density and sensitivity are improved by applying such an intermediate gelatin layer between the support and the emulsion layer.
However, known methods of preventing pressure marking when used in photographic elements including tabular silver halide grains have proved less effective. In particular, when the hardening degree is increased to achieve a very low swelling index and to improve its resistance to pressure desensitization, photographic characteristics are reduced. Accordingly, the problem still remains of preventing pressure marking in photographic elements including light-sensitive tabular silver halide emulsions.
U.S. Pat. No. 4,414,304 describes forehardened photographic elements, particularly radiographic elements, including at least one hydrophilic colloid emulsion layer containing tabular silver halide grains having an aspect ratio of not lower than 5:1 and a projective area of not lower than 50%. The elements require no additional hardening on development and give images of high covering power. Among gelatin hardeners, bis(vinylsulfonylmethyl) ether, mucochloric acid and formaldehyde are described.
Japanese Pat. Appl. No. J5 9105-636 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains, the binder of at least one of the hydrophilic colloidal layers being gelatin which has jelly strength of at least 250 g. Wet coat strength of said elements is improved without reducing covering power.
Japanese Pat. Appl. No. J6 2249-140 describes photographic elements comprising at least one silver halide emulsion layer containing tabular silver halide grains and halogen substituted s-triazine type hardeners. The elements are suitable for rapid processing and have improved pressure resistance.
U.S. Pat. No. 4,847,189 describes a photographic element comprising at least one silver halide emulsion layer containing tabular silver halide grains with an aspect ratio not lower than 5:1 and showing a melting time of from 8 to 45 minutes. The melting time and the gelatin amount of the element renders the element suitable for rapid processing of 45 sec. and improves the pressure desensitization resistance.
EP 238,271 discloses a silver halide photographic element comprising at least one hydrophilic colloidal layer on a support, showing a melting time of from 8 to 45 minutes, and a water content of from 10 to 20 g/m.sup.2 upon completion of the washing step. The element is preferably processed in a developing solution comprising indazole and benzotriazole derivatives. The preferred processing time is 45 sec.
U.S. Pat. No. 4,647,528 discloses a method of increasing both covering power and scratch resistance by using a particular polymeric hardener in a photographic element comprising a support coated with least one silver halide emulsion layer containing tabular silver halide grains with an aspect ratio higher than 5:1.
As above mentioned silver halide emulsion layers are coated on a polymeric film support. In particular, photographic elements which require accurate physical characteristics use polyester film bases, such as polyethyleneterephthalate film bases and cellulose ester film bases, such as cellulose triacetate film bases. Silver halide radiographic elements are generally composed of a polyethyleneterephthalate electrically insulating support and silver halide emulsion layers coated thereon. Such a structure promotes the formation and accumulation of static charges when subjecting the radiographic elements to friction or separation, caused by contact with the surface of the same or different elements during steps for manufacturing of the photographic elements or when using them for photographic purposes.
These accumulated static charges cause several drawbacks, which are more evident when the radiographic film is manufactured and/or processed at high speed. The most serious drawback is discharge of accumulated charges prior to development processing, by which the light-sensitive silver halide emulsion layer is exposed to light to form dot spots or branched or feathery linear specks when development of the photographic film is carried out. This is the phenomenon of the so-called "static marks". Such static marks cause a reduction of the commercial value of photographic films, which sometimes become completely useless. For example, the formation of static marks in medical or industrial X-ray films may result in a very dangerous judgment or erroneous diagnosis. Static marks are a particular problem because it becomes evident for the first time by carrying out development. Further, these static charges are also the origin of secondary problems such as adhesion of dusts to the surface of films, uneven coating, and the like.
As mentioned above, such static charge are frequently accumulated when manufacturing and/or processing silver halide photographic elements. For example, during production, they are generated by friction of the photographic film contacting a roller or by separation of the emulsion surface from the support surface during a rolling or unrolling step. Further, they are generated on X-ray films in an automatic apparatus by contact with or separating from mechanical parts or fluorescent screens, or they are generated by contact with or separation from rollers and bars made of rubber, metal, or plastics in a bonding machine or an automatic developing machine or an automatic developing apparatus or in a camera in the case of using color negative films or color reversal films. In addition they can be generated by contacting with packing materials, and the like.
Additionally, photographic elements comprising light-sensitive layers coated onto polymeric film bases, when stored in rolls or reels which are mechanically wound and unwound or in sheets which are conveyed at high speed, tend to accumulate static charges and record the light generated by the static discharges.
Silver halide photographic elements having high sensitivity and handling speed are subject to an increase of static mark appearance. In particular, static marks are easily generated because of high sensitization of the photographic element and severe handling conditions such as high speed coating, high speed exposure, and high speed automatic processing.
Other drawbacks which result from the accumulation of electric charges on polymeric film bases are the adherence of dust and dirt, coating defects and limitation of coating speed.
The static-related damages occur not only before the photographic element has been manufactured, exposed and precessed, but also after processing when the photographic element including the image is used to reproduce and enlarge the image. Accordingly, it is desired to provide permanent antistatic protection which retains its effectiveness even after processing.
In order to prevent problems caused by static charges, it is suitable to add an antistatic agent to the silver halide photographic elements. However, antistatic agent conventionally used in other fields cannot be used freely for silver halide photographic elements, because they are subjected to various specific restrictions due to the nature of the photographic elements. More specifically, the antistatic agents which can be used in silver halide photographic elements must have excellent antistatic abilities while not having adverse influences upon photographic properties of the photographic elements, such as sensitivity, fog, granularity, sharpness. Further, such antistatic agents must not have adverse influences upon the film strength and upon antiadhesion properties. Furthermore, the antistatic agents must not accelerate exhaustion of processing solutions and not deteriorate adhesive strength between layers composing the silver halide photographic element.
In the art of silver halide photographic elements a wide number of solutions to the above described problems have been suggested in patent and literature references, mainly based on charge control agents and electrically conductive compounds coated on the silver halide emulsion layer together with a binder as an antistatic layer.
The most useful charge control agents known in the art are ionic and non-ionic surfactant as well as ionic salts. Fluorinated surfactants are often mentioned as good antistatic agents in silver halide photographic elements.
Electrically conductive compounds are capable of transporting charges away from areas where they are not desired. Typical examples of such electrically conductive substances are polyelectrolites such as the alkali metal salts of polycarboxylic acids or polysulfonic acids, or quaternary ammonium polymers, which dissipate the electrical charge by providing a surface which conducts electrons by an ionic mechanism. However, such compounds are not very suitable in antistatic layers because they lose effectiveness under conditions of low relative humidity, become sticky under conditions of high relative humidity, and lose their antistatic effect after passage through processing baths.
It is known in the art that preferred antistatic materials are those that conduct electrons by a quantum mechanical mechanism rather that an ionic mechanism. This is because antistatic materials that conduct electrons by a quantum mechanical mechanism are effectively independent of humidity. They are suitable for use under conditions of low relative humidity, without losing effectiveness, and under conditions of high relative humidity, without becoming sticky. Defect semiconductor oxides and conductive polymers have been proposed as electronic conductors which operate independent of humidity. A major problem, however, with such electronic conductors is that they generally cannot be provided as thin, transparent, relatively colorless coatings by solution coating methods. The use of vanadium oxide has proved to be the one exception. That is, effective antistatic coatings of vanadium oxide can be deposited in transparent, substantially colorless thin films by coating from aqueous dispersions.
It is known to prepare an antistatic layer from an aqueous composition comprising vanadium oxide as described, for example, in FR Patent Application No. 2,277,136, BE Patent No. 839,270, U.S. Pat. No. 4,203,769 and GB Patent Application No. 2,032,405. The composition comprising the vanadium oxide may contain a binder to improve mechanical properties of an antistatic layer produced therefrom, such as cellulose derivatives, polyvinyl alcohols, polyamides, styrene and maleic anhydride copolymers, copolymer latexes of alkylacrylate, vinylidene chloride and itaconic acid. It is also known to provide such vanadium oxide antistatic layers with a protective overcoat layer that provides abrasion protection and/or enhances frictional characteristics, such as a layer of cellulosic material.
In photographic elements, the antistatic layer comprising vanadium oxide can be located on the side of the film base opposite to the image-forming layer as outermost layer, with or without a protective abrasion-resistant topcoat layer, of can be located as a subbing layer underlying a silver halide emulsion layer or an auxiliary gelatin layer. As vanadium oxide can diffuse from the antistatic layer through the overlying protective layer or gelatin layer into the processing solutions, a diminution or loss of the desired antistatic protection results.
U.S. Pat. No. 5,006,451 describes a photographic element comprising a film base having thereon an antistatic layer comprising vanadium oxide and a barrier layer which overlies the antistatic layer and is comprised of a latex polymer having hydrophilic functionality. This patent reports that said barrier layer prevents the vanadium oxide from diffusing out of the underlying antistatic layer and thereby provides permanent antistatic protection. However, the solution provided by said patent requires a two layer construction which requires additional investment and operating cost, and has been proved by experiments that it looses antistatic protection in processing solutions such as developing and fixing solutions.
Japanese Pat. Appl. No. J05/119433 describes a plastic base film for silver halide photographic material having a layer of polymer binder and vanadium pentoxide coated on at least one side of said plastic base. However, the plastic base is subjected to a tenter treatment after the layer of polymer binder and V.sub.2 O.sub.5 is coated thereon.
In summary, there is still the need for a silver halide radiographic element which allows a high handling speed, both during manufacturing and processing, without the occurrence of static marks and worsening of physical and photographic properties.